1. Field of the Invention
The present invention relates to a method of forming a forsterite insulating film on the surface of a grain-oriented silicon steel sheet.
2. Description of the Prior Art
As a method of forming an insulating film on the surface of a grain-oriented silicon steel sheet, there has generally known a method, wherein a silicon steel sheet is cold rolled into a given final gauge, the cold rolled steel sheet is decarburized at a temperature of 700.degree.-900.degree. C. under a wet hydrogen atmosphere, to form a sub-scale containing silica on the surface of the steel sheet, an annealing separator consisting mainly of magnesia is applied to the steel sheet surface, and the above treated steel sheet is wound up in the form of a coil and then subjected to a final annealing to form a forsterite insulating film on the surface of the silicon steel sheet.
The forsterite insulating film formed in the above described method has a high influence upon the appearance of the final product. When the film is ununiform, the product cannot be supplied to the market, and therefore the film has a high influence upon the production yield of the product. Particularly, magnesia used as a main component of the annealing separator has a very high influence upon the property of an insulating film, which will be formed during the final annealing, and therefore a large number of investigations have hitherto been carried out.
For example, Japanese Patent Application Publication No. 3,726/66 takes a notice of the particle size of primary particle of magnesia to be used as an annealing separator, and discloses a method, wherein a slurry of magnesia having a particle size of the primary particle of 170-280 A is applied to the surface of a silicon steel sheet, and a substantially pure magnesium oxide film tightly adhered to the steel sheet surface is formed. Further, Japanese Patent Application Publication No. 14,162/70 discloses a method, which uses magnesia obtained by firing magnesium hydroxide containing not larger than 0.2% of impurity in two stages of low temperature and high temperature, and containing at least 70% of particles having a particle size of not larger than 3.mu..
In addition to the above described methods, various proposals have been made with respect to annealing separator. Some of the proposed annealing separators are effective, but they are expensive in the commercial scale mass production. Moreover, since there are various factors having an influence upon the property of annealing separator, the obtained result often lacks reproducibility, and it has been impossible to prevent completely the formation of film, which cannot be used for practical use.